A brake pressure control valve, particularly a trailer control valve, to control the brake pressure for a trailer vehicle pulled by a tractor.
Such a brake pressure control valve or trailer control valve is described in U.S. Pat. No. 4,493,510. With the trailer control valve of the prior art, it is possible during a braking action triggered by an activation of the brake valve on the tractor vehicle, that brake pressure will increase in the brake system of the trailer vehicle in advance of the brake pressure increase in the tractor vehicle. During a braking action, therefore, the tractor vehicle and the trailer vehicle will be kept stretched apart.
When the brake valve of the tractor vehicle activated, a relay piston in the trailer control valve is pressurized with a control pressure. By means of a stroke of the relay piston which then takes place, an inlet valve activated by the relay piston is opened. The braking system of the trailer vehicle is thereby connected with a pressure medium source. The trailer control valve thereby delivers a pressure which causes a pressure increase in the brake system of the trailer vehicle.
A pressure increase in the brake system of the trailer vehicle which is in advance of that in the tractor vehicle occurs when, at the beginning of the braking process, the maximum active surface area of the relay piston subject to the control pressure is pressurized, so that there is a pressure increase of the pressure delivered to the trailer control valve which is in advance of the braking application itself.
During the further course of the braking application, a further increase in the control pressure acting on the relay piston results in the opening of an inlet valve located in the relay piston, which connects a control chamber located above the relay piston with a control chamber located underneath the relay piston. The control chamber located underneath the relay piston is delineated by an annular active surface of the relay piston, which acts in opposition to the above-mentioned maximum active surface of the relay piston.
There is a pressure adjustment between the upper and the lower sides of the relay piston, in which the pressure delivered by the trailer control valve acting on the relay piston also participates. The ratio of the control pressure existing during the pilot phase to the pressure delivered by the trailer control valve is also changed, i.e., the phase of the advance is terminated, and the higher control pressure now required equals the control pressure required for the braking phase itself of the trailer vehicle. This delivery pressure advance produced in the pilot phase can be regulated.
The inlet valve located in the relay piston, consisting of a valve seat and a disc valve element, is located in a valve body, and can be moved in its longitudinal position in the relay piston. The valve body is attached to the relay piston by a spring. A valve tappet located in the relay piston projects against the disc valve element of the inlet valve in such a way that with the displacement of the valve body in the relay piston, which takes place under the action of the control pressure, the valve tappet raises the disc valve element from its valve seat. The above-mentioned connection of the two control chambers to one another, corresponding to the relay piston, is thereby established.
The distance between the valve tappet and the disc valve element is adjustable, i.e., the distance from the end of the valve tappet to the disc valve element can be varied by the distance the valve tappet is screwed into the relay piston by means of a threaded connection.
As a result of the control pressure introduced into the trailer control valve during a braking action, the valve body executes a stroke against the force of the above-mentioned spring in the relay piston, before the disc valve element of the inlet valve is raised from its seat by the valve tappet and thus the inlet valve is opened.
If, for example, the valve tappet is set so that the stroke is only small until the opening of the inlet valve, then the spring force to be overcome is also small. Accordingly, even with a relatively low control pressure, the pilot phase is ended. This means that even the advance pressure delivered by the trailer control valve is correspondingly low. Conversely, with a relatively large stroke of the valve body before the opening of the inlet valve, the advance pressure delivered is correspondingly higher.
On the trailer control valve mentioned above, the setting stroke of the valve tappet is limited in the direction of the inlet valve. The limitation is imposed by a stop corresponding to the relay piston, which prevents the valve tappet from being screwed into the relay piston beyond a specified adjustment stroke. As a result of this limitation, the minimum advance control pressure of the trailer control valve is determined.
The adjustment of the delivery pressure is generally made under load. In this case, the control inlet is connected with a control pressure sensor, and the pressure inlet is connected with a pressure medium source. The pressure to be delivered can be verified by means of a pressure measurement system, which is connected to the pressure outlet. Such an adjustment is made at the factory on a test stand. However, it can also be made on the vehicle under conditions similar to those on the test stand. The latter is necessary if, for example, the present advance brake pressure set at the factory must be adjusted to the braking force action of the trailer vehicle.
If, for example, on the occasion of a client readjustment on the vehicle, a greater advance delivery pressure is desired at the vehicle, then with the trailer control valve mentioned above, the valve tappet can easily be retracted, that is, screwed out of the relay piston to a position in which the inlet valve is spaced further from the tappet so that greater piston movement is required to open the valve. This could result in an undesirable, rapid increase of the pressure delivered to the trailer control valve. During vehicle operation, this would result in an undesirable overbraking of the trailer vehicle, which might not be immediately noticeable under relatively favorable conditions. In a critical braking situation however, e.g., on a slick road, it could lead to an abrupt locking of the braked wheels, and to the trailer vehicle veering out of line.
The object of the invention, therefore, is to improve a trailer control valve of the type described above, so that the advance brake pressure delivered to the trailer vehicle by the trailer control valve can be limited to a specified maximum pressure level.
This object is achieved by the invention described in Patent claim 1. Refinements and advantageous embodiments of the invention are described in the subclaims.
The invention has the advantage that with with an adjustment of the advance pressure to be delivered by the trailer control valve, either in the factory on a test stand or on the vehicle under conditions similar to those on the test stand, a defined maximum pressure level can be specified as the starting pressure for the beginning of regulation. In this manner, the trailer control valve is prevented from being placed in a control status in which there could be an extreme over-control of the trailer.
Another advantage of the invention is that an adjustment on the vehicle can be made even under conditions which are not similar to those on the test stand, e.g., when the brake system is depressurized, without any risk of the above-mentioned overbraking.
Another advantage results from the shape of the above-mentioned valve tappet, whereby in any position of the valve tappet, a uniform opening stroke of the inlet valve connecting the control chambers of the relay piston is achieved.